1. Field
A method of bonding metal balls for magnetic head assemblies that bond electrode pads of sliders and those of flexible printed circuit boards is provided.
2. Description
A magnetic head assembly used in a hard disk drive (HDD) is formed of a slider into which a magnetoresistive element is incorporated and a thin flexible metallic plate, and includes a flexure for elastically supporting the slider and a flexible printed circuit board. The flexible printed circuit board is bonded on the surface of the flexure, and electrically connects the magnetoresistive element of the slider and a circuit system of a device to which this magnetic head assembly is attached. The flexure is fixed to a load beam by, for example, spot welding.
Conventionally, the electrode pads of the magnetoresistive element of the slider and those of the flexible printed circuit board in such a magnetic head assembly have been bonded using gold balls such that the electrode pads are disposed orthogonal to each other. Since the bonding area (the size of the electrode pads and gaps therebetween) has decreased as the sizes of the magnetic head assembly and the slider have been reduced in recent years, solder-ball bonding using solder balls that have smaller diameters than gold balls has been proposed instead of gold-ball bonding.
In solder-ball bonding, a solder ball is fixed on the electrode pad of the slider or on the electrode pad of the flexible printed circuit board using flux. The solder ball is then heated such that the electrode pad of the slider and the electrode pad of the flexible printed circuit board are bonded by the melted solder. According to this form of solder-ball bonding, the joint between the electrode pad of the slider and the electrode pad of the flexible printed circuit board can easily be removed by heating the bonded portion. The flexure can easily be recycled even when the magnetic head assembly is determined to be unacceptable in static and dynamic characteristic tests performed before shipment (See Japanese Unexamined Patent Application Publication Nos. 5-29404, 2000-12598, 2002-25025 (US Publication No. 20020179696), and 2002-45962 (U.S. Pat. No. 6,336,581)).
However, the above-described solder-ball bonding requires flux that fixes the solder ball placed on a bonding surface, and therefore is not applicable to parts that can be contaminated by flux. Various bonding methods without using flux have been proposed recently.
For example, a solder ball is adsorbed by an adsorption nozzle and placed on a bonding surface. While the solder ball is mechanically positioned by the adsorption nozzle, part of the solder ball is melted by a first laser irradiation so as to be temporarily fixed. Then, the adsorption nozzle is removed from the solder ball, and the entire solder ball is melted by a second laser irradiation so as to be completely fixed. The power of laser beams cannot be higher than a predetermined level since the laser beams are applied to the solder ball while the solder ball is mechanically retained by the adsorption nozzle. Therefore, the solder ball cannot be entirely melted by one laser irradiation, and the laser beams are required to be applied to the solder ball in two steps of temporary fixing and final fixing. Thus, the number of steps is increased, and the possibility of heat damage is also increased due to the two laser irradiations.
In another method, the solder ball is placed on the bonding surface by the adsorption nozzle, and then is melted by one laser irradiation while the solder ball is pressed against the bonding surface using nitrogen gas instead of the adsorption nozzle. In this method, the solder ball moves after the solder ball is placed on the bonding surface, and the bonding position is difficult to accurately control.
In another method, the solder ball dropped from a carrying route to the bonding surface is pressed against the bonding surface by nitrogen gas flowing from the same carrying route, and is melted by laser beams emitted through the same carrying route while the solder ball is pressed against the bonding surface. In this method the supplying paths of the solder ball and the laser beams are coaxial, and a high-power laser such as an yttrium aluminum garnet (YAG) laser must be used such that a long focal length of the laser beams can be ensured. Moreover, focusing of the laser beams is difficult since the guiding path of the laser beams is complicated.